Powder coating of non-crystalline and crystalline epoxy resins

ABSTRACT

Provided are low temperature curing thermosetting epoxy powder coating compositions comprising at least one non-crystalline epoxy resin, at least one crystalline epoxy resin, a curing agent, and a cure catalyst. Coatings made from such powder compositions exhibit a substantial reduction in bubble entrapment, enough to prevent a visible haze from forming in the coating, when cured at low temperatures demanded by brass substrates, while the powders from which the coatings are made still exhibit good storage stability and melt-processability.

FIELD OF THE INVENTION

This invention relates to a thermosetting epoxy powder coatingcomposition and, more particularly, to an epoxy powder coatingcomposition adapted to prevent bubble entrapment within the coating filmformed therefrom during thermal curing at low temperatures.

BACKGROUND OF THE INVENTION

Brass is a widely used material of construction for many articles ofcommerce. Parts formed from brass generally require a clear coat finishto enhance their luster and appearance and achieve protection againstwear or the environment. Solvent-borne liquid coatings have been triedfor such purposes, but they fail to deliver the needed performance.Solvent-borne coatings also contain alarmingly high levels of hazardousvolatile organic compounds which tend to flash away during curing,making it necessary to contain and collect the vapor of volatileingredients which is rather costly.

Fusion bonded, thermosetting powder coating compositions have also beenused to coat brass parts. Powder coatings offer a number of advantagesover liquid coatings. For instance, corrosion and scratch resistance ismuch superior to that of liquid coatings. In addition, powder coatingsare virtually free of the harmful fugitive organic solvents normallypresent in liquid coatings and, accordingly, give off little, if any,volatiles during curing, which eliminates solvent emission problems anddangers to the health of workers employed in coating operations.

Because brass parts, for example, brass plated zinc die cast parts, aresusceptible to outgassing upon heating, thermosetting powder coatingsthat are capable of curing at low temperatures, e.g., below about 350°F., are generally preferred to minimize substrate outgassing duringcuring and permanent scarring of the finish coating. Low curetemperatures are also desired, since brass tends to discolor or tarnishat higher temperatures.

Among the commercially available low temperature cure thermosettingpowder coatings, GMA acrylic powder coatings have been the most widelyused by the brass finishing industry. GMA acrylics offer coatings withexceptional smoothness and clarity, but also suffer from a number ofdrawbacks including poor adhesion and rather high cost which is becomingincreasingly difficult for the brass finishing industry to bear.Attempts have been made to replace the GMA acrylics with traditionalthermosetting epoxy powder coatings, such as those based on standardnon-crystalline epoxy resins, e.g., bisphenol A type epoxy resins,standard curing agents for epoxy resins, e.g., dicyanodiamide, alongwith standard catalysts, e.g., 2-methyl imidazole. While epoxies offerimproved adhesion and reduced cost, curing of these coatings at thedesired low temperature cure conditions, generally leads to bubbleentrapment within the finish coating, which is particularly troublesomefrom an appearance and film quality standpoint.

Bubble entrapment is believed to occur during powder application. Inparticular, as the powder coating is applied to the substrate, it isbelieved that air is entrapped within the powder particles as they aredeposited on the substrate. When the powder begins to melt, flow andeventually cure, the air must escape from the coating before the finishcoating hardens or it will be trapped as tiny bubbles dispersedthroughout the coating. With traditional thermosetting epoxy powdercoatings, the latter effect tends to occur at the desired lowtemperature cure conditions, which is believed to be due, at least inpart, to the rather high melt viscosities experienced at suchtempartures. In a clear coating, those bubbles are especiallyproblematic in that they tend to create an unwanted hazy appearance thatinterferes with the distinctness of image of the finish coating, i.e.,the sharpness of image reflected by a coating's surface. Brass coatings,however, should have a high and visually consistent distinctness ofimage to enable one to see through the finish coating as if looking at apolished brass part.

What is needed is a thermosetting epoxy powder coating compositionadapted to cure at low temperatures below about 350° F. as well asprevent bubble entrapment within the finish coating during heat curingat said low temperatures.

SUMMARY OF THE INVENTION

It is, therefore, a primary object of this invention to provide athermosetting epoxy powder coating composition that does not suffer fromthe forgoing drawbacks.

It is another object of this invention to provide a thermosetting epoxypowder coating composition adapted to cure at low temperatures as wellas prevent bubble entrapment and resultant visible haze within thefinish coating during curing at said low temperatures.

It is yet another object of this invention to provide a thermosettingepoxy powder coating composition that has a relatively low viscositywhen melted for curing, so that it can readily flow, coalesce and degasat low temperatures before reaching a thermoset state.

It is a further object of this invention to provide a thermosettingepoxy powder coating composition of the aforesaid character that isstill melt-processable in an extruder under standard conditions andstorage stable at ambient temperatures.

It is a related object of this invention to provide a coated substrate,preferably a clear coated brass substrate, having a thermosetting powdercoating composition of the aforesaid character coated and cured thereonwhile the substrate is at a temperature generally below the substrateoutgassing and/or degradation temperature, with the resulting finishcoating, remarkably, having little or no bubble entrapment and resultantvisible haze therein.

The foregoing objects of the invention are accomplished through animprovement in a low temperature thermosetting epoxy powder coatingcomposition, especially in a clear composition that is essentially freeof opaque pigments and fillers. The powder coating composition inaccordance with this invention comprises a film-forming particulateblend of a non-crystalline epoxy resin, a curing agent for the epoxyresin, and a cure catalyst. The improvement being that small amounts ofa crystalline epoxy resin are included in the composition to improve thelow temperature degassing properties of the composition and thus reduceor eliminate bubble entrapment within the coating film formed therefromupon curing, particularly at the rather low cure temperatures demandedby certain substrates, especially those made of brass, yet withoutdetracting from the ability of the composition to be melt processed andstored in a conventional manner. The thermosetting powder coatingcomposition of this invention is, therefore, useful in providingessentially haze-free coatings on substrates demanding lower curetemperatures, such as brass parts. Our discovery herein that theaddition of small amounts of crystalline resin prevents bubbleentrapment at low cure temperatures in the above formulation was trulyunexpected. There is no indication of this unexpected benefit in theliterature. U.S. Pat. No. 5,414,058 (Ono) makes no mention ofimprovements in powder degassing. Moreover, we are not aware of anyprior art relating to blends of non-crystalline and crystalline epoxyresins for use in thermosetting powder coatings which achieve the novelfilm properties described herein.

In accomplishing the foregoing objects of this invention, there is alsoprovided a method for obtaining an essentially haze-free coating,preferably a clear coating, having little or no optical bubble defectson a substrate surface susceptible to outgassing and/or degradation uponheating at elevated temperatures, preferably on a surface made of brass,comprising applying a low temperature powder coating composition of theaforesaid character on a substrate surface, and curing the powdercoating on the substrate at temperatures sufficient to cure the powdercoating composition and below the outgassing and/or degradationtemperature of the substrate. Also provided by this invention is asubstrate susceptible to outgassing and/or degradation upon heating,preferably a brass substrate, coated, preferably clear coated, with anessentially haze-free layer of a cured thermosetting epoxy powdercoating composition which in its uncured state comprises the compositionof the aforesaid character.

The various objects, features and advantages of this invention willbecome more apparent from the following description and appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Throughout this specification, all parts and percentages specifiedherein are by weight unless otherwise stated. Also herein, thenon-crystalline epoxy resin component plus crystalline epoxy resincomponent are considered to be the "resin system" and equal to 100parts. Levels of other components are calculated as parts relative to100 parts of the resin system ("phr"). Further, as used herein, the term"non-crystalline epoxy" (otherwise referred to as "amorphous epoxy")broadly defines epoxy resins which show no, or trace, crystallization ormelting point as determined by differential scanning calorimetry (DSC).While the term "crystalline epoxy" used herein denotes crystalline aswell as semi-crystalline materials and broadly defines epoxy resins witha discernable crystallization or melting point by DSC.

According to the present invention, there are provided thermosettingepoxy powder coatings that have improved low temperature degassingproperties and yield essentially bubble-free and haze-free when cured atsaid low temperatures, while the powders from which the coatings aremade still have good storage stability and melt-processability. Theepoxy powder coating compositions of this invention typically include anon-crystalline epoxy resin, a curing agent, and a catalyst, with theimprovement being that a small amount of crystalline epoxy resinsufficient to prevent bubble entrapment within the cured coating filmformed therefrom is included in the resin system of the composition inplace of some non-crystalline epoxy resin. By "bubble-free", what ismeant is that trapped bubbles are reduced to such an extent that avisible haze cannot be discerned by the unaided human eye in thecoating. It is possible that closer inspection may reveal tiny bubbleswithin the coating; however, they are not sufficient to interfere withthe aesthetic appearance and desired clarity of the coating as requiredby conventional standards.

The non-crystalline epoxy resins useful herein include, withoutlimitation, bisphenol A type epoxy resins which are diglycidyl ethers ofbisphenol A, usually produced by the reaction of epichlorohydrin andbisphenol A. The bisphenol A type epoxy resins used herein arepreferably solid resins which have a Tg by DSC of greater than about 40°C., so that the powders made from such resins are storage stable,preferably greater than about 55° C., an epoxy functionality of about 2or greater, preferably about 2 to 4, and epoxide equivalent weight ofabout 600 to 1100, preferably about 600 to 750.

In accordance with the present invention, the non-crystalline epoxyresin is blended with a sufficient amount of crystalline epoxy resin toreduce or eliminate bubble entrapment upon low temperature curing. Thecrystalline epoxy resins useful herein are preferably solid resins whichhave a Tm by DSC of greater than the Tg of the non-crystalline resin,preferably greater than about 90° C., so that the powders made from suchresins can be melt-processed in a conventional extruder without a)causing substantial processing delays while waiting for the crystallineresins to recrystallize and b) substantial reduction in the storagestability of the powder as a result of destruction of crystalstructures, an epoxy functionality of about 2 or greater, preferablyabout 2 to 3, and an epoxide equivalent weight of about 50 to 500,preferably about 100 to 300. Such resins can be produced by conventionaltechniques, such as by reacting epichlorohydrin with a di- or polyhydricmonomer known to have limited rotation with epichlorohydrin.

Examples of suitable crystalline epoxy resins include, withoutlimitation, prepolymers selected from the group consisting oftetramethylbisphenol diglycidyl ether, bisphenol S diglycidyl ether,2,5-di-t-butylbenzene-1,4-diglycidyl ether, hydroquinone diglycidylether, 2,5-di-t-butylhydroquinone diglycidyl ether, terephthalic aciddiglycidyl ether, diglycidyl isophthalate, triglycidyl isocyanurate, andepoxypropoxydimethylbenzyl acrylamide, which are also described in U.S.Pat. No. 5,414,058, the teaching of which is incorporated by referenceherein in its entirety. Tetramethylbisphenol diglycidyl ether is aparticularly preferred crystalline epoxy resin in this invention.

The amount of crystalline substance incorporated in the powder coatingcomposition is critical to the success of this invention. Only athreshold minimum amount of the above crystalline resin is required toproduce the desired results. While this critical lower limit can varydepending upon the particular crystalline substance employed, ingeneral, as little as about 5% by weight of crystalline resin, based onthe total weight of crystalline and non-crystalline epoxy resin, will besufficient to reduce or eliminate bubble entrapment within the coatingfilm when cured at the desired low temperatures. Generally, it has beenfound that below about 5% by weight crystalline resin, effective bubblereduction cannot be attained in most cases. Amounts greater than thisminimum level of crystalline resin can, of course, be employed, althoughat some point the increase in low temperature degassing properties issubstantially outweighed by a substantial decrease inmelt-processability and shelf stability of the powder composition suchthat the powder composition is rendered impractical for commercial use.Generally, it has been found that when the crystalline resin exceeds anupper limit of about 15% by weight, the storage stability of the powdercoating, in particular, tends to become extremely poor as a result ofsevere blocking at ambient temperatures, rendering the powderimpractical for commercial use. Therefore, in a preferred embodiment ofthis invention, the amount of crystalline epoxy resins is generallyabout 5-15% by weight, based on the total weight of the crystalline andnon-crystalline epoxy resins.

A curing agent is typically incorporated in the powder coating of thisinvention to crosslink the epoxy resins at the epoxy sites and providethe desired thermoset properties to the coating, although it is possibleto cure the coating without a curing agent. The curing agents usefulherein are preferably solid materials having at least two functionalgroups reactive with the epoxy groups. Examples of suitable curingagents include, without limitation, dicyanodiamide, bisphenol A,bisphenol S, bisphenol A epoxy adduct of an aliphatic polyamine having aprimary or secondary amino group, with dicyanodiamide being preferred.Generally, the curing agent is used in an amount of 0.7-1.7 equivalents,preferably 1.1-1.4 equivalents of the functional group per oneequivalent of the epoxy group present in the powder coating composition.Typically, this translates to a range of about 3 to 7 phr of curingagent in the powder coating composition, preferably about 4.5 to 5.5phr.

Although it is possible to cure or crosslink the powder coating withoutthe use of catalysts, it is usually desirable to employ a cure catalystin the powder coating composition of this invention to permit the curingreaction to progress at commercially acceptable rates. The curecatalysts useful herein are preferably solid materials known to promotean epoxy ring opening function and the formation of ether linkagesbetween epoxy resins. Particularly preferred catalysts include, withoutlimitation, 2-methyl imidazole, 2-phenyl imidazole, as well as bisphenolA epoxy adducts of the aforesaid imidazoles if lower temperatures/fastercures are desired. Generally, the amount of catalysts employed in thepowder coating ranges from about 0.01 to 0.3 phr, preferably about 0.05to 0.1 phr.

The powder coating composition of this invention may be clear, i.e.,unpigmented or unfilled, or contain standard pigments and fillers toimpart the desired color and opacity to the coating film, although thebenefits of this invention are most effectively achieved in clearformulations. By "clear", it is meant that the powder coatingcomposition is essentially free of opaque pigments and fillers, so thatit will produce cured coating films that are essentially transparent.

In addition to the above components, the thermosetting powder coatingcomposition of this invention may contain the usual additives such as,without limitation, standard dry flow additives, flow control agents,leveling agents, degassing agents, antioxidants, uv absorbers, lightstabilizers, etc.

The cure temperatures of the above powders will vary somewhat dependingon the various ingredients employed. However, it is particularlyimportant that the coating powders possess the ability to cure at lowtemperatures without trapping bubbles within the cured coating filmformed therefrom. Substrates susceptible to outgassing and/ordegradation upon heating, such as brass parts, generally require a curetemperature below about 350° F. In accordance therewith, it is requiredthat the powder coating of this invention be formulated to cure to athermoset state at temperatures- below about 350° F., preferably betweenabout 325° F. and 350° F., within commercially reasonable times, e.g.,30 minutes or less, preferably 15 minutes or less, while still producingessentially bubble-free and haze-free coating films. Preferably, thepowder coating composition of this invention will experience little orno bubble entrapment throughout the entire cure schedule.

Powder coatings of this invention are prepared in the usual manner.First, an intimate mixture is formed by dry blending together all of theformulation ingredients in a mixer. The dry blend is then melt-blendedin a mixing extruder with heating above the melting point of the resinand other ingredients, where necessary, so that the extrudate is athorough and homogeneous mixture. Extrusion is preferably carried out attemperatures either below or close to the Tm of the crystalline epoxyresin for efficient melt-processing and desired storage stability.Gaseous or supercritical fluid, e.g., CO₂, may be charged to theextruder for better control of the extrusion temperatures. Thereafter,the extruded composition is rapidly cooled and solidified and thenbroken into chips. Next, the chips are ground in a mill with cooling,and, as necessary, the particulates are screened and sorted according tosize. Average particle size desired for electrostatic application isgenerally between about 20 and 60 microns. Once the dry, free-flowing,powders of this invention, which now contain at least onenon-crystalline epoxy resin and at least one crystalline epoxy resin,are produced, they are ready for application onto a substrate to becoated.

The powder coatings of this invention can then be applied to thesubstrate by any conventional powder coating technique, althoughelectrostatic application, e.g., electrostatic spraying, is generallypreferred. In electrostatic spray coating, electrostatic spray boothsare normally employed which house banks of corona discharge ortriboelectric spray guns and a reclaim system for recycling theoverspray powders into the powder feed. The substrate is heated, atleast on the surface, at the time of application and/or subsequently toa temperature equal to or above the temperature needed to cure thepowder coating and below the substrate outgassing and/or degradationtemperature, so that the coating particles sufficiently melt, flow andform a smooth continuous coating film, and then cure to a thermosetstate without degrading the substrate. Heating can be performed ininfrared, convection ovens, or a combination of both, although infraredovens are preferred. Time and temperature of the final cure will varysomewhat depending on the coating powders employed and conditions ofuse. However, regardless of cure time and temperatures employed,provided that the powder ingredients have been sufficiently meltedbefore curing, the coating films generated on the substrates will have avisually consistent appearance and will be without entrapped bubblesthat interfere with the aesthetic appearance and distinctness of imagerequired by conventional standards.

The powder coating compositions are particularly suited for applicationonto metallic substrates, particularly brass, susceptible to outgassingand/or degradation upon heating. Since the above powders are adapted tocure at relatively low temperatures, they are also suited forapplication onto other types of heat sensitive substrates, such as woodsubstrates, e.g., hardwood, hard board, laminated bamboo, woodcomposites, particle board, electrically conductive particle board,high, medium or low density fiber board, masonite board, and othersubstrates that contain a significant amount of wood, as well asplastics, e.g., ABS, PPO, SMC, BMC, polyolefins, polycarbonates,acrylics, nylons and other copolymers which usually will warp or outgaswhen coated and heated with traditional heat curable powders, along withpaper, cardboard, and heat resistant metallic composites and componentshaving a metallic or non-metallic heat sensitive aspect and possiblyhaving a variable mass. The coatings of this invention are also suitedfor typical heat resistant substrates, such as high temperature metals,steels, and other alloys, glass, ceramic, carbon and graphite.

This invention will now be described in greater detail by way ofspecific examples.

EXAMPLE 1 Clear Epoxy Powder Coatings Derived From 0, 1, 5, 10, and 20Parts Crystalline Resin

The following ingredients were melt-blended together in the manner givenin the Table below to provide powder coatings derived respectively from0, 1, 5, 10 and 20 parts crystalline epoxy and 100, 99, 95, 90 and 80parts non-crystalline epoxy to demonstrate the improvement in bubbleentrapment provided by this invention.

    ______________________________________                                                            PHR                                                       INGREDIENTS           A      B     C   D   E                                  ______________________________________                                        DRY BLEND IN A KNEADER UNTIL HOMOGENEOUS                                        Araldite GT 7013.sup.1 (Non-Crystalline Epoxy)                                                        100    99  95  90  80                                 Epon RSS 1407.sup.2 (Crystalline Epoxy) 0 1 5 10 20                           Dihard 100S.sup.3 (Curing Agent) 5 5 5 5 5                                    2-Methyi Imidazole (Catalyst) 0.1 0.1 0.1 0.1 0.1                             Baysilone Oil (Leveling Agent) 0.4 0.4 0.4 0.4 0.4                            Benzoin (Outgassing Agent) 0.8 0.8 0.8 0.8 0.8                                Tinuvin 900.sup.4 (UV Absorber) 2 2 2 2 2                                     Tinuvin 622.sup.5 (UV Absorber) 1 1 1 1 1                                   THEN MELT BLEND IN A TWIN SCREW EXTRUDER At 180° F.                      COOL EXTRUDATE AND THEN BREAK INTO CHIPS                                      CHARGE CHIPS AND 0.5% ALUMINUM OXIDE C.sup.6 TO A                             BRINKMANN MILL                                                                THEN GRIND TO POWDER AND SCREEN TO -200 Mesh                                ______________________________________                                         Table Footnotes                                                               .sup.1 Araldite GT 7013 is a bisphenol A type epoxy resin having a Tg of      55° C., an epoxy equivalent weight of 650-725, and an epoxy            functionality of 1.9 to 2, sold by Ciba Specialty Chemicals.                  .sup.2 Epon RSS 1407 is a tetramethylbisphenol diglycidyl ether               crystalline epoxy resin having a Tm of 108° C., an epoxy equivaien     weight of 166, and an epoxy functionality of 2, sold by Shell Chemical.       .sup.3 Dihard 100S is a dicyanodiamide curing agent, sold by SKW Chemical     .sup.4 Tinuvin 900 is a benzotriazole uv absorber, sold by Ciba Specialty     Chemicals.                                                                    .sup.5 Tinuvin 622 is a benzotriazole uv absorber, sold by Ciba Specialty     Chemicals.                                                                    .sup.6 Aluminum Oxide C is a fumed alumina dry flow additive, sold by         Sullivan Associates.                                                     

Each powder formulation listed above was electrostatically sprayed witha corona discharge gun onto separate polished brass panels in an amountsufficient to obtain a 2.5-3.0 mil thick dry film on the panel uponcuring. After powder application, for bubble testing purposes, thepanels were cured first at 250° F. for 5 minutes, allowed to cool toambient temperature, and then cured further at 325° F. for 3 minutes togenerate a clear coating film on the substrate. These panels were thenevaluated for bubbles and film clarity. The performance results for theindividual coating powders and the coating films formed therefrom aregiven in the Table below.

    ______________________________________                                        PROPERTY A       B       C     D       E                                      ______________________________________                                        Clarity.sup.1                                                                          Bubbles Bubbles No    No Bubbles                                                                            No Bubbles                                  Bubbles                                                                    Blocking.sup.2 4 4 4 6 10                                                   ______________________________________                                         Table Footnotes                                                               .sup.1 The clarity data for each sample was gathered using a Nikon optica     microscope at 200 times magnification through which bubbles were observed     No bubbles means that virtually no bubbles were observed.                     .sup.2 The blocking data for each sample was gathered by placing the          powder in a test tube and subjecting the powder to a 100 g weight for 24      hours at 140° F. Powders are rated on a scale of 1 to 10, with 1       being free flowing powder after 24 hours, and 10 being fully sintered         material. Blocking resistance of about 6 or below is commercially             acceptable.                                                              

The results demonstrate the range over which the approach of theinvention is effective to eliminate bubble entrapment for the powdercoatings. At 1% of crystalline resin, the coating still contains bubblesafter cure. At 5% and above, the coatings are bubble free. At 20%crystalline resin, the coating displays extreme blocking which would notmake it practical for commercial use.

EXAMPLE 2 Clear Epoxy Powder Coatings Derived From 0 and 5 PartsCrystalline Resin

The following ingredients were melt-blended together in the same manneras in Example 1 to provide powder coatings derived respectively from 0and 5 parts crystalline epoxy and 100 and 95 parts non-crystalline epoxyto further demonstrate the utility of this invention.

    ______________________________________                                                             PHR                                                      INGREDIENTS             F      G                                              ______________________________________                                        Araldite GT 7013 (Non-Crystalline Epoxy)                                                              100    95                                               Epon RSS 1407 (Crystalline Epoxy) 0 5                                         Casamine OTB.sup.1 (Curing Agent) 5 5                                         2-Phenyl Imidazole (Catalyst) 0.05 0.05                                       Baysilone Oil (Leveling Agent) 0.4 0.4                                        Benzoin (Outgassing Agent) 0.8 0.8                                            Tinuvin 900.sup.4 (UV Absorber) 2 2                                           Tinuvin 622.sup.5 (UV Absorber) 1 1                                           Aluminum Oxide C (Post Blend) 0.5 0.5                                       ______________________________________                                         Table Footnotes                                                               .sup.1 Casamine OTB is 1(o-tolyl)biguanide curing agent, sold by Swan         Chemical.                                                                

RESULTS

The individual coatings were then tested in the same manner as inExample 1. The results are given in the Table below.

    ______________________________________                                        PROPERTY       F       G                                                      ______________________________________                                        Clarity        Bubbles No Bubbles                                               Blocking 4 4                                                                ______________________________________                                    

The above results confirm that incorporation of at least 5% crystallineepoxy resin into the powder coating compositions eliminates bubbleentrapment and prevents a haze from visibly developing on the surface ofthe coating upon being cured at temperatures safe for brass parts.

From the foregoing it will be seen that this invention is one welladapted to attain all ends and objects hereinabove set forth togetherwith the other advantages which are apparent and inherent. Since manypossible variations may be made of the invention without departing fromthe scope thereof, the invention is not intended to be limited to theembodiments and examples disclosed, which are considered to be purelyexemplary.

Accordingly, reference should be made to the appended claims to assessthe true spirit and scope of the invention, in which exclusive rightsare claimed.

What is claimed is:
 1. A thermosetting epoxy powder coating compositionin particulate form comprising a film-forming blend of:a) at least onenon-crystalline epoxy resin; b) from at least about 5% to about 15% byweight of at least one crystalline epoxy resin in an amount sufficientto reduce or eliminate bubble entrapment within the cured coating filmformed from the powder coating composition; and, c) a curing agent tofacilitate curing of said composition to a thermoset state,whereinincorporation of said crystalline epoxy resin reduces or eliminatesbubble entrapment within the cured coating film formed from the powdercoating composition when said composition is cured to a thermoset stateat temperatures below about 350° F.
 2. The composition of claim 1,wherein said crystalline epoxy resin is incorporated in an amountsufficient to reduce or eliminate bubble entrapment within the curedcoating at temperatures below about 350° F., while in an amountinsufficient to cause the blocking resistance of said composition to bereduced to commercially impractical levels.
 3. The composition of claim1, wherein said composition is a clear composition being essentiallyfree of opaque pigments and fillers.
 4. The composition of claim 1,wherein said at least one non-crystalline epoxy resin has a Tg ofgreater than about 40° C.
 5. The composition of claim 1, wherein said atleast one crystalline epoxy resin has a Tm of greater than about 90° C.6. The composition of claim 1, which further comprises a catalyticamount of a cure catalyst.
 7. The composition of claim 6, wherein thecure catalyst in 2-methyl imidazole.
 8. The composition of claim 1,wherein said crystalline epoxy resin is selected from the groupconsisting of tetramethylbisphenol diglycidyl ether, bisphenol Sdiglycidyl ether, 2,5-di-t-butylbenzene-1,4-diglycidyl ether,hydroquinone digylcidyl ether, 2,5-di-t-butyl hydroquinone diglycidylether, terephthalic acid diglycidyl ester, diglycidyl isophthalate,epoxypropoxydimethylbenzylacrylamide, and triglycidyl isocyanurate. 9.The composition of claim 1, wherein said crystalline epoxy resin istetramethylbisphenol diglycidyl ether.
 10. The composition of claim 1,wherein said non-crystalline epoxy resin is a bisphenol A epoxy resin.11. The composition of claim 1, wherein the curing agent isdicyanodiamide.
 12. A thermosetting epoxy powder coating composition inparticulate form comprising a film-forming blend of:a) at least onenon-crystalline epoxy resin having a Tg of greater than about 40° C.; b)from at least about 5% to about 15% by weight of at least onecrystalline epoxy resin having a Tm of greater than about 90° C. in anamount sufficient to reduce or eliminate bubble entrapment within thecured coating film formed from the powder coating composition; and, c) acuring agent to facilitate curing of said composition to a thermosetstate;wherein incorporation of said crystalline epoxy resin reduces oreliminates bubble entrapment within the cured coating film formed fromthe powder coating composition when said composition is cured to athermoset state at temperatures below about 350° F.
 13. The compositionof claim 12, wherein said non-crystalline epoxy resin is a bisphenol Aepoxy resin, said crystalline epoxy resin is tetramethylbisphenoldiglycidyl ether, said curing agent is dicyandiamide, said compositionfurther comprises an imidazole cure catalyst, and said composition isessentially free of opaque pigments and fillers so as to form a clearcoating film upon curing.